1. Field of the Invention
This invention relates generally to filled fluoropolymeric composites. More particularly, this invention relates to ceramic-filled fluoropolymeric composites for use as electrical substrate materials, comprising high temperature, high modulus polymeric powders. The substrate material exhibits a high flexural modulus, as well as good dimensional stability. Such substrate materials are especially useful as laminates for manufacturing microwave circuits.
2. Brief Description of the Prior Art
Fluoropolymer (predominantly PTFE) matrix composites are widely used as substrates in the microwave frequency circuit board industry. PTFE composites have excellent electrical properties, high temperature resistance, and outstanding solvent resistance. Many of the general types of fluoropolymeric circuit board substrates are described in "Specification for Plastic Substrates, Clad or Unclad for High Speed/High Frequency Interconnections" (Document IPC-L-125). The eight general types are described in sections IPC-L-125/01 to IPC-L-125/08.
Fluoropolymer substrate types 1 to 5 are composites of PTFE and glass fiber. Types 1, 2 and 5 are reinforced with woven glass fabric, while types 3 and 4 are reinforced with non-woven E-glass fibers. PTFE/glass fiber composites are low dielectric constant (K') materials, with the dielectric constant ranging from 2.15 to 2.65. These types of materials exhibit a relatively high flexural modulus due to the glass fiber reinforcement, but are limited in the range of dielectric constants achievable (K' less than about 3.0). Furthermore, these materials often exhibit a comparatively high dielectric loss, due to the loss properties of the glass fabric. Most of these materials also exhibit a comparatively high z-axis coefficient of thermal expansion (CTE) (greater than 100 ppm/.degree.C.) since the glass fabric provides reinforcement predominantly only in the XY plane.
Fluoropolymer substrate types 6, 7, and 8 are PTFE composites filled with ceramic powder in order to alter the dielectric properties of the substrate. The dielectric constant of these types of materials can be tailored over a comparatively wide range by varying the type of ceramic filler. Comparatively low dielectric loss may be obtained through use of low loss fillers such as fused amorphous silica. It is also possible to make low z-axis CTE (less than 50 ppm/.degree.C.) circuit substrates, since the ceramic filler reduces the CTE of the composite more isotropically than glass fabric. Low z-axis CTE is desirable since it improves the reliability of plated through hole connections if the boards are subjected to thermal cycling.
Examples of these ceramic powder-filled composites include Rogers Corporation's RO3000.RTM. product line. This is a family of low dielectric loss silica/titania powder-filled PTFE composite microwave circuit substrates, available with dielectric constants of 3.0, 6.15, and 10.2. The RO3000.RTM. products also exhibit excellent dimensional stability (DimStab) and low z-axis CTE independent of dielectric constant, since the total filler contents are approximately the same. The differing dielectric constants are achieved by varying the ratio of silica to titania filler. These composite materials are described in commonly-assigned application No. 08/283,302, the disclosure of which in herein incorporated by reference in its entirety.
Other ceramic-filled composites have been described in U.S. Pat. No. 4,996,097 to Fischer. Fischer generally discloses a PTFE matrix with from 25 to 85 volume percent particulate filler and an optional organic polymeric thermoset resin to lower lamination temperatures and improve adhesion of the film. U.S. Pat. No. 5,545,475 to Korleski discloses a microfiber-reinforced porous polymer film and method of manufacture. The microfibers are produced by the addition of liquid crystalline polymer (LCP) powder to PTFE, paste extrusion, calendering, and then stretching the resulting sheet at high temperature to stretch the LCP particles into fibrils. Neither of these aforementioned patents address the problem of increasing the flexural modulus of particulate-filled fluoropolymer composites.
While these materials are suitable for the various uses for which they are intended, among circuit board processors accustomed to flexurally stiff woven glass reinforced materials, there is a perceived need for ceramic powder-filled materials stiffer than those currently available. The "high flexibility" (low flexural modulus) of these type 6, 7, and 8 materials can lead to difficulties in material handling, particularly with conveyorized equipment designed to operate with glass fabric reinforced circuit substrates, and particularly at laminate thicknesses of less than about 0.020".